Steam cooling apparatus for gas turbine

ABSTRACT

The present invention relates to a steam-cooling system for gas turbine combustors in combined plants possessing a steam-cooled combustor; in order to maintain the combustor outlet steam temperature at planned values even during the starting and during the periods of load fluctuation, a steam valve 12 is opened and steam from a boiler 4 is conducted, and thereby the combustor of the gas turbine 1 is cooled, and steam recovery valve 11 is opened and this steam is returned to steam turbine 5. During the starting, steam is caused to flow from an auxiliary steam source 3, and a warming up is conducted. When the amount of steam at the combustor outlet decreases, the temperature of the temperature sensor 31 increases, and when this exceeds a planned value, a temperature regulating valve 30 is opened and steam from steam turbine 5 is extracted and is let into the cooling steam outlet side flow path 61 of the combustor. Furthermore, when the value of a pressure sensor 24 reaches a predetermined value, a bypass valve 14 is opened and steam is caused to flow to a condenser 6, and thus the necessary amount of steam is maintained within the combustor. This is all controlled by means of the controller 10, which constantly maintains the temperature of the cooling steam at planned values.

This application is a 371 of PCT/JP98/02801 filed Jun. 24, 1998.

TECHNICAL FIELD

The present invention relates to a device for steam cooling thecombustor of a gas turbine which, in a combined plant in which a gasturbine and a steam turbine are combined, is capable of accuratelycontrolling the temperature of the steam at planned temperatures evenduring periods of load change.

BACKGROUND ART

FIG. 7 is a schematic diagram of a plant having a gas turbine combustorwhich is subjected to steam cooling, in a combined plant in which a gasturbine and a steam turbine are combined. In the figure, a combustiongas 7, which is created as a result of power generation in gas turbine 1and is discharged, is supplied to the boiler 4, and in boiler 4, steam 9is generated by this high-temperature combustion gas 7 from the gasturbine 1, and the exhaust gas 50 is discharged to the atmosphere fromsmokestack 51. The steam 9, which is generated in the boiler 4, issupplied to steam turbine 5, and this turns a power generator, so thatelectric power is obtained. The cooling of the combustor of gas turbine1 is conducted by extracting a portion of the steam produced by boiler 4and conveying this steam 40 to the combustor, and the steam heatedduring this cooling process as recovered steam 41 is then reused bybeing returned to steam turbine 5.

Next, the control of the steam cooling for the gas turbine combustor ina combined plant having the structure described above will be explained.

FIG. 6 is a schematic flow diagram of a steamcooling system for gasturbine combustors in a conventional combined plant. In the figure,controller 2 controls the flow of the steam, while combustion gas fromthe gas turbine 1 is led to the boiler 4, which generates steam.Furthermore, this steam-cooling system is provided with an auxiliarysteam source 3, a steam turbine 5, and a condenser 6. Steam recoveryvalve 11 is provided in the recovered steam flow path 61 from the outletof the combustor of gas turbine 1. Furthermore, steam valve 12 isprovided in extracted steam flow path 62 from the boiler 4 to the inletof the combustor of gas turbine 1. Auxiliary steam valve 13 is providedin flow path 63 in order to introduce the steam from the auxiliary steamsource 3 into the flow path 62 leading to the inlet of the combustor ofgas turbine 1. The opening and closing of these valves 11 through 13 iscontrolled by the controller 2.

Furthermore, a temperature sensor 21, which detects the temperature ofthe steam flowing through the auxiliary steam flow path 63, atemperature sensor 22, which detects the temperature of the steamflowing into the inlet of the combustor of gas turbine 1, and atemperature sensor 31, which measures the temperature of the steam atthe outlet of the combustor of gas turbine 1, are provided in thesystem, and the detected values detected by these temperature sensorsare input into controller 2. In addition to the parts described above,an actual plant would be provided with a drain exhaust system, openingand closing valves, flow rate and pressure adjustment valves, pressuredetectors, and the like; however, as these are not required for anexplanation of the technological background of the present invention, anexplanation thereof will be omitted here.

In a control system such as that described above, prior to supplyingsteam to the combustor of the gas turbine 1, the warming of the pipingsystems, and the discharge of the drain during operation, are conducted;however, those systems are omitted from the figures. Prior to starting,the auxiliary steam valve 13 is first opened, and auxiliary steam isallowed to flow into the auxiliary steam flow path 63 from auxiliarysteam source 3, and this flows through the combustor of gas turbine 1via flow path 62, and is discharged via a flush pipe which is notdepicted in the figure, so that a warming up is conducted. Next, gasturbine 1 is started, and after a predetermined period of time, theauxiliary steam valve 13 is closed, while steam valve 12 and steamrecovery valve 11 are opened, the steam extracted from the boiler 4 issupplied to the combustor of gas turbine 1, and the combustor is cooledusing this steam, while the steam heated in the process of cooling isreturned to steam turbine 5 and reused. The amount of cooling steamsupplied to the combustor of gas turbine 1 is adjusted to the amountnecessary for the gas turbine load by conducting programmed control inthe controller 2.

As described above, when the signals of temperature sensors 21, 22, and31 are input into the controller 2, the opening and closing of theauxiliary steam valve 13, the steam valve 12, and the steam recoveryvalve 11 is conducted in accordance with a program predetermined, andsteam cooling is conducted so that the combustor of gas turbine 1remains at planned temperatures from the starting of the gas turbine andthroughout the operation thereof.

As described above, in conventional steam-cooling systems for gasturbine combustors in combined plants, the combustor is cooled usingsteam extracted from a boiler, and after it has been used for cooling,the steam is returned to the steam turbine as recovered steam, and therequired amount of steam is controlled by a controller using a programdetermined in advance in accordance with the load on the gas turbine.However, during the starting in the plant, and during periods of loadchange, delays occur in response to the temperature and pressure of thesteam generated at the boiler, and as a result of these delays, thesteam employed for cooling the combustor is insufficient, so that thereare cases in which the steam temperature in the flow path on the coolingsteam outlet side of the combustor increases, and exceeds plannedtemperatures, so that the temperature of the combustor increasesexcessively. Furthermore, in order to counteract this insufficiency inthe steam employed for cooling the combustor, it was necessary toincrease the size of the boiler.

DISCLOSURE OF INVENTION

The present invention has as an object thereof to provide asteam-cooling system for gas turbine combustors which, in combinedplants having a steam-cooled combustor, is capable of maintainingplanned temperatures by preventing an excessive rise in steamtemperature in the gas turbine combustor even during the starting ofplant or during periods of load change.

The invention of the present application comprises a steam-coolingsystem for gas turbine combustors in combined plants, in whichcombustion gas exhausted from a gas turbine is directed to a boiler,steam is generated in this boiler and a steam turbine is operated usingthis steam while a portion of the steam from the boiler is extracted andsupplied to the combustor of the gas turbine to cool the combustor, andafter being used for this cooling, the steam is returned to the steamturbine; which comprises: a temperature sensor for detecting thetemperature of steam in a cooling steam outlet side flow path of the gasturbine combustor; a steam flow path through which steam is extractedfrom the exhaust system of the steam turbine and the extracted steamfrom the exhaust system of the steam turbine is let into the coolingsteam outlet side flow path of the gas turbine combustor via atemperature regulating valve; and a controller which receives detectedtemperature signals from the temperature sensor and conducts controlsuch that the valve is opened when the detected temperature is in excessof a predetermined value, and closed the valve when this temperature isequal to or lower than the predetermined value.

The controller conducts control such that when the temperature of thesteam at the cooling steam outlet side of the gas turbine combustorexceeds a planned temperature set in advance, the temperature regulatingvalve opens. When the temperature detected by the temperature sensorexceeds the planned value, the controller which receives this detectedtemperature signal controls opening operation of the valve, and thisallows low-temperature steam extracted from the exhaust system of thesteam turbine to enter into the cooling steam output side flow path ofthe gas turbine combustor, thus regulating temperature in such a way asto decrease the temperature of the steam flowing in the cooling steamoutlet side flow path. Next, when the temperature of the steam in theflow path at the cooling steam outlet side of the combustor reaches theplanned value, the valve is closed, and normal control resumes. By meansof such control, it is possible to prevent an excessive rise in thesteam temperature in the flow path at the cooling steam outlet side ofthe gas turbine combustor even during the starting of the plant orduring fluctuations in the load, and it is thus possible to control thetemperature so that it remains at planned values.

Another mode of the invention of the present application involves asteam-cooling system for gas turbine combustors, comprising a pressuresensor for detecting a difference in pressure between the inlet sideflow path and the outlet side flow path of the cooling steam of thecombustor, and a bypass flow path for allowing an outflow of steam fromthe cooling steam outlet side flow path of the combustor to thecondenser via a bypass valve; a detected temperature signal from thetemperature sensor and a pressure signal from the pressure sensor areinput into the controller, and the controller conducts control such thatwhen the detected temperature is higher than a predetermined value, thetemperature regulating valve is opened, while when this temperature isequal to or lower than the predetermined value, the valve is closed, andfurthermore conducts control such that when the pressure is lower than apredetermined value, the bypass valve is opened, while when thepredetermined value is reached, the bypass valve is closed.

In the structure described above, when, as a result of some cause, astate is reached in which the amount of steam used for cooling isinsufficient and the necessary amount of steam is not caused to flow,there are cases in which the difference in pressure detected by thepressure sensor remains below the predetermined value even if an inflowof low-temperature steam is caused by the temperature regulating valve.In such cases, the controller controls opening operation of the bypassvalve, the cooling steam outlet side flow path of the combustor isplaced in communication with the condenser, and the pressure differencebetween the cooling steam inlet side flow path and outlet side flow pathof the combustor is forcibly increased, so that steam is caused to flow,and thus it is possible to prevent an excessive increase in thetemperature of the steam flowing through the cooling steam outlet sideflow path of the gas turbine combustor even during the starting of plantor during periods of load fluctuation, and thus to conduct control atplanned values.

Another mode of the invention of the present application comprises asteam-cooling system for gas turbine combustors, wherein, in the statein which the temperature regulating valve is opened, if the temperaturedetected by the temperature sensor is not reduced to a predeterminedvalue, the controller conducts control so as to open the bypass valve.

In the structure described above, the temperature at the cooling steamoutlet side flow path of the combustor is detected, so that thecontroller first controls opening operation of the temperature controlvalve, and in cases in which this control is insufficient, next controlsopening operation of the bypass valve. Furthermore, in cases in whichthe difference in pressure between the cooling steam inlet side flowpath and outlet side flow path of the combustor is low, this bypassvalve is opened irrespective of the steam temperature in the vicinity ofthe outlet of the combustor. Accordingly, using both values detected bythe pressure sensor and the temperature sensor, the controller controlsthe bypass valve, so that the reliability of control is improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic flow diagram showing a steam-cooling system forgas turbine combustors in accordance with an embodiment of the presentinvention.

FIG. 2 is a timing chart of the control of the steam-cooling system forgas turbine combustors in accordance with an embodiment of the presentinvention.

FIG. 3 is a schematic flow diagram of a steam-cooling system for gasturbine combustors in accordance with a further embodiment of thepresent invention.

FIG. 4 is a flow chart of the characteristic portions of the controllerof the present invention in a steam-cooling system for gas turbinecombustors in accordance with a further embodiment of the presentinvention.

FIG. 5 is a timing chart of the control of the steam-cooling system forgas turbine combustors in accordance with a further embodiment of thepresent invention.

FIG. 6 is a schematic flow diagram of a conventional steam-coolingsystem for gas turbine combustors.

FIG. 7 is a schematic diagram of a combined plant which is provided witha conventional steam-cooled combustor.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinbelow, embodiments of the present invention will be explainedconcretely based on the figures.

FIG. 1 is a schematic flow diagram of a steam-cooling system for gasturbine combustors in accordance with an embodiment of the presentinvention. In FIG. 1, references 1, 3 through 6, 11 through 13, and 21through 22 have functions identical to those in the conventional exampleshown in FIG. 6, and a detailed description thereof will be omittedhere, and they simply will be described by reference. Furthermore, thecharacteristic parts of the present invention are those given referencenumbers 10, 30, and 31, and these will be described in detailhereinbelow.

In FIG. 1, controller 10 conducts control so as to open the auxiliarysteam valve 13 prior to the starting, similar to the above-mentionedconventional example, and auxiliary steam flows from auxiliary steamsource 3 into auxiliary steam flow path 63, and the steam is led to thecombustor of gas turbine 1 via flow path 62, and is discharged via aflush pipe which is not depicted in the figure, so that a warming up isconducted. Next, the starting of gas turbine 1 is conducted, and after apredetermined period of time, auxiliary steam valve 13 is closed, andsteam valve 12 is simultaneously opened, and steam recovery valve 11 isalso opened and the extracted steam from boiler 4 is supplied to thecombustor of gas turbine 1, the combustor is cooled, and after beingused for cooling, the steam is returned to steam turbine 5 via coolingsteam outlet side flow path 61. As a concrete example of this case, thesteam from boiler 4 is extracted from the outlet of an IPSH(intermediate pressure super heater), and the recovered steam isreturned to an HTR (high-temperature steam reheater).

The control described above is similar to that of the conventionalexample shown in FIG. 6; however, the present invention is furtherprovided with the following functions.

Controller 10 controls the amount of steam required for combustorcooling in accordance with the starting of plant or during changes inthe load during periods of load fluctuation; however, such control doesnot immediately reflect the pressures and temperatures of the steamgenerated in the boiler, but rather a delay is produced, and as a resultof this delay, there are cases in which the cooling steam for thecombustor is insufficient and the steam temperature at the combustoroutlet is in excess of the planned temperature.

The temperature signal of temperature sensor 31 is input at thecontroller 10, and when the detected temperature exceeds a plannedtemperature which is set in advance, the controller 10 conducts controlsuch that the temperature valve 30 is opened. By means of opening thetemperature valve 30, the exhaust gas from steam turbine 5, that is tosay, low-temperature reheated steam, is extracted, and this is injectedinto the flow path 61 at the recovered steam side, that is to say, thecooling steam outlet side of the combustor. By means of this controller10, the temperature of the steam at the combustor outlet side, which hadbecome high, is regulated and the temperature thereof decreases, andwhen this reaches the planned temperature, control is conducted so thatthe temperature regulation valve 30 is closed, and the control of normaloperations is resumed.

FIG. 2 is a timing chart of the control in an actual embodiment of theexplanation above. In the figure, the uppermost level indicates thepatterns of the rotation and load of gas turbine 1; the load of gasturbine 1 increases slowly from a period of 30 minutes after thestarting, and at some point after the passage of 150 minutes it reaches100%. The controller 10, in accordance with the load pattern, maintainsauxiliary steam valve 13 in an open state prior to the starting of gasturbine 1 and for a period of more than 60 minutes after the startingthereof; the auxiliary steam from auxiliary steam source 3 is let intoflow path 62. Furthermore, the amount of steam necessary for thecombustor after the supply of auxiliary steam is also set in accordancewith this load pattern. The controller 10 opens steam valve 12 and steamrecovery valve 11 simultaneously with the closing of auxiliary steamvalve 13; by allowing steam from boiler 4 to enter flow path 62 inaccordance with the pattern of the amount of steam required for thecombustor, the combustor is cooled. After it is used for cooling, thissteam is returned to steam turbine 5 via steam recovery valve 11.

Furthermore, the recovered steam temperature (combustor outlettemperature) of the temperature sensor 31 experiences a transition atthe planned temperature; however, when there is a sudden increase in theload at some point after the passage of 150 minutes, as a result of thedelay in the supply of steam and the like, the steam temperature risesabove the planned temperature T. At this time, the controller 10 opensthe temperature regulating valve 30, and the exhaust gas from steamturbine 5, that is to say, the low-temperature reheated steam, isextracted, and this is let into the cooling steam outlet side flow path61 of the combustor and the temperature is regulated, and when thetemperature returns to the planned value, the temperature regulatingvalve 30 is closed, and normal control is resumed.

In accordance with the above embodiment of the present invention, in asteam-cooling system for gas turbine combustors, a temperature sensor 31and a temperature regulating valve 30 are provided in order to preventan excessive increase in the steam temperature at the outlet of thecombustor, and control is exerted by the controller 10, and a portion ofthe exhaust gases of the steam turbine 5 are extracted and returned tothe combustor outlet side, so that it is possible to control thetemperature at the outlet of the combustor of gas turbine 1 at plannedvalues even during the starting of plant or during periods of loadchange, and furthermore, in order to respond to insufficiencies in thesteam used for combustor cooling, it is not necessary to increase thesize of the boiler.

FIG. 3 is a schematic flow diagram of a steam-cooling system for gasturbine combustors in accordance with another embodiment of the presentinvention; those parts having identical reference numbers as in FIG. 1have the same function. A temperature sensor 23 for detecting the steamtemperature at the combustor outlet is provided in the vicinity of thecombustor outlet in the flow path 61 at the cooling steam outlet side ofthe combustor, and a bypass valve 14 is placed in the flow path 64 (thebypass path) which leads to the condenser from the vicinity of theoutlet of the combustor in flow path 61 at the cooling steam outlet sideof the combustor, and the values detected by the temperature sensor 23are transmitted to the controller 10. Furthermore, a pressure sensor 24for detecting the difference in pressure between the steam inlet sideflow path 62 and the steam outlet side flow path 61 of the combustor isinstalled between flow path 61 and flow path 62, and the value detectedthereby is transmitted to controller 10.

Controller 10 conducts the following control, which is a characteristicfeature of the present invention. In other words, when the pressuredifference detected by pressure sensor 24 at the combustor outlet of gasturbine 1 is small, the necessary amount of steam is not flowing to thecombustor, so that the temperature of temperature sensor 31 alsoincreases, and in such a case, in order to guarantee the necessaryamount of steam to the combustor, controller 10 controls openingoperation of the bypass valve 14, and control is conducted so that steamflows to condenser 6 via bypass path 64. In this way, it is possible toprevent an excessive heating of the combustor by means of forcing adifference in pressure between the combustor outlet and inlet andcausing a flow of steam.

Next, as a second embodiment, in the case in which the temperature oftemperature sensor 31 is high, the temperature regulation valve 30 isfirst opened, and if control cannot be effected in this way, then thebypass valve 14 is also opened. Furthermore, when the difference inpressure between the cooling steam inlet side flow path 62 and outletside flow path 61 of the combustor is low, then this bypass valve 14 isopened irrespective of the steam temperature of the cooling steam outletside flow path 61 of the combustor detected by temperature sensor 31.Here, the temperature may be detected using the temperature sensor 23,which is provided in the vicinity of the combustor outlet in the coolingsteam outlet side flow path 61 of the combustor, in place of thetemperature sensor 31.

FIG. 4 is a flow chart showing, among the control conducted bycontroller 10, the parts which are characteristic of the presentinvention. In S1, the cooling of the combustor by steam is conducted,steam valve 12 is opened and steam is led from boiler 4 to thecombustor, the combustor is cooled, and the steam is recycled to steamturbine 5 via steam recovery valve 11.

In S2, during cooling, if the temperature detected by temperature sensor31 increases and becomes greater than a prespecified temperature, thetemperature of the combustor increases, and the steam used for coolingwill be insufficient, so that a determination is made as to whether thecombustor temperature is increasing, and in S3, temperature regulationvalve 30 is opened, and low-temperature steam from steam turbine 5 isinjected into the cooling steam outlet side flow path 61 of thecombustor.

In S4, cases are observed in which, irrespective of the fact that in S3low-temperature steam was injected into the steam flow path of thecombustor for a predetermined period of time by means of temperatureregulating valve 30, the steam flow rate to the combustor isinsufficient, and the temperature of the recovered steam increases. Thatis to say, a determination is made as to whether the temperaturedetected by temperature sensor 31 is higher than the temperaturedetected by temperature sensor 31 in S2, and when it is higher, bypassvalve 14 is opened for a predetermined period of time in S5, steam fromthe cooling steam outlet side flow path 61 of the combustor is caused toflow to the condenser 6, and the pressure difference between the coolingsteam inlet side flow path 62 and outlet side flow path 61 of thecombustor is forcibly increased and steam is caused to flow, thuspreventing the overheating of the combustor. Here, the temperature inthe vicinity of the outlet of the combustor may be detected using thetemperature sensor 23 in place of the temperature sensor 31, and adetermination may be made in S2 as to whether this temperature is higherthan the temperature detected by temperature sensor 31, and the samecontrol may be conducted.

In S6, in the case in which the temperature detected by temperaturesensor 31 in S2 above is unchanged, or in S7, in the case in which thedetected value is unchanged, the cooling of the combustor is continued.

In S7, further, a determination is made as to whether the difference inpressure of the pressure sensor 24 is lower than a predeterminedpressure, and when it is lower, the bypass valve 14 is opened in S8, andsteam is caused to flow to the condenser 6.

FIG. 5 is a timing chart of the control in the embodiment of theexplanation above. In the figure, the uppermost level shows the patternof the rotational speed and load of the gas turbine 1; the load of gasturbine 1 exhibits a pattern such that it slowly increases after aperiod of 30 seconds from starting, and reaches a level of 100% at apoint after the passing of 150 minutes. Controller 10, in accordancewith this load pattern, maintains the auxiliary steam drive 13 in anopened state from before the starting of gas turbine 1 to a point intime after the passage of 60 minutes or more from the starting, andcauses an inflow of auxiliary steam from the auxiliary steam source 3into flow path 62. Furthermore, the amount of steam necessary for thecombustor after the supply of this auxiliary steam is also set inaccordance with the load pattern.

Controller 10 controls opening operation of steam valve 12 and steamrecovery valve 11 simultaneously with the closing of auxiliary steamvalve 13, and by means of causing an inflow of steam from boiler 4 intoflow path 62 in accordance with the pattern of the necessary amount ofsteam for the combustor, the combust or is cooled. After being used forcooling, the steam is recycled to steam turbine 5 via steam recoveryvalve 11.

Furthermore, the recovered steam temperature (combustor outlettemperature) of temperature sensor 31 experiences a transition at aplanned temperature at a point up to 150 minutes after the starting;however, during the rapid increase in the load after 150 minutes ofoperation, as a result of the delay in the supply of steam and the like,the steam temperature exceeds the planned temperature T. At this time,controller 10 controls opening operation of temperature regulating valve30, and the exhaust gas from steam turbine 5, that is to say, thelow-temperature reheated steam, is extracted, and this is injected intothe cooling steam outlet side flow path 61 of the combustor, and thetemperature is regulated, and when the temperature return s to theplanned value, the temperature regulating valve 30 is closed, and normalcontrol is resumed. Up to this point, the operation is identical to thatestablished in FIGS. 1 and 2.

Here, when the difference in pressure between the inlet side flow path62 and the outlet side flow path 61 of the cooling steam of thecombustor, which is detected by pressure sensor 24, is lower than apredetermined value, bypass valve 14 is opened irrespective of the steamtemperature at the cooling steam outlet side flow path 61 of thecombustor detected by temperature sensor 31.

In accordance with a further embodiment of the present invention, in asteam-cooling system for gas turbine combustors, a temperature sensor 31and a temperature regulating valve 30 are provided in order to preventan excessive increase in the steam temperature at the combustor outlet,and by means of controller 10, a portion of the steam discharged fromsteam turbine 5 is extracted and this is returned to the combustoroutlet. Furthermore, in addition to this control, temperature sensor 23,pressure sensor 24, and bypass valve 14 are provided and the steamoutputted by the combustor is caused to flow out to condenser 6. It ispossible to control the outlet temperature of the combustor of gasturbine 1 at planned values even during the starting of plant and duringperiods of load change, and furthermore, it is not necessary to installa larger boiler in order to address insufficiencies in the steam used tocool the combustor.

INDUSTRIAL APPLICABILITY

In accordance with the structure described above, when the steamtemperature in the cooling steam outlet side flow path of a combustor ofa gas turbine increases, low-temperature steam discharged from the steamturbine discharge system is extracted, and this is injected into thecooling steam outlet side flow path, and it thus becomes possible toregulate the steam temperature, and furthermore, even if for some reasonthe amount of cooling steam decreases, tending to lead to an increase inthe temperature of the combustor, the cooling steam outlet side flowpath of the combustor is placed in communication with the condenser byopening a bypass valve, and the difference in pressure between thecooling steam inlet side flow path and outlet flow path of the combustoris increased, and control is conducted such that steam is caused toflow, so that it is possible to control the steam temperature in thecooling steam outlet side flow path of the combustor of a gas turbine atplanned values without an excessive increase therein even during thestarting or during periods of load change.

Furthermore, in the state in which the temperature regulating valve isopened, if the temperature detected by the sensor drops below apredetermined value, the controller opens the bypass valve, and thereby,in addition to the value detected by the pressure sensor, control isconducted while detecting the temperature of the cooling steam outletside flow path of the combustor as well, so that the reliability ofcontrol is increased.

What is claimed is:
 1. A steam-cooling system for gas turbine combustorsin combined plants, in which combustion gas exhausted from a gas turbineis directed to a boiler, steam is generated in the boiler, and a steamturbine is driven using this steam while a part of the steam from theboiler is extracted and supplied to the gas turbine combustor to coolthe combustor, and after being used for this cooling, the steam isreturned to the steam turbine; which comprises:a temperature sensor fordetecting the temperature of steam in a cooling steam outlet side flowpath of the gas turbine combustor; a steam flow path through which steamis extracted from an exhaust system of the steam turbine and theextracted steam from the steam turbine is let into the cooling steamoutlet side flow path of the gas turbine combustor via a temperatureregulating valve; and a controller which receives detected temperaturesignals from the temperature sensor and conducts control such that thevalve is opened when the detected temperature is in excess of apredetermined value, and closed when this temperature is equal to orlower than the predetermined value.
 2. A steam-cooling system for gasturbine combustors in accordance with claim 1, comprising a pressuresensor for detecting a difference in pressure between an inlet side flowpath and the outlet side flow path of the cooling steam of thecombustor, and a bypass flow path for allowing an outflow of steam fromthe cooling steam outlet side flow path of the combustor to a condenservia a bypass valve; anda detected temperature signal from thetemperature sensor and a pressure signal from the pressure sensor areinput into the controller, and the controller conducts control such thatwhen the detected temperature is higher than a predetermined value, thetemperature regulating valve is opened, while when this temperature isequal to or lower than the predetermined value, the valve is closed, andfurthermore conducts control such that when the detected pressure islower than a predetermined value, the bypass valve is opened, while whenthe predetermined value is reached, the bypass valve is closed.
 3. Asteam-cooling system for gas turbine combustors in accordance with claim2, wherein, in the state in which the temperature regulating valve isopened, if the temperature detected by the temperature sensor is notreduced to a predetermined value, the controller conducts control so asto open the bypass valve.